Many mechanical devices exist for carrying out angular positioning. For example, a simple step motor can permit angular positioning according to one of its steps. However, the conventional devices can be unsuitable for certain applications. This is the case for example when angular precisions of approximately a hundredth of a degree are necessary. Furthermore, additional difficulties are encountered when it is wished to obtain good stability and good reliability of the different angular positions. In other words, it is difficult to obtain precise positions which can be maintained for given durations, and which can be obtained once more when they have been left. Stringent requirements in terms of precision, stability and reliability apply notably in the field of optical instrumentation. By way of example, an optical instrument may need calibration of one of its sensors. This calibration can consist of orienting a mirror around an axis according to three distinct positions. A first position, known as the scene position, corresponds to the position in which the mirror reflects radiation from an area to be studied. It is said that the mirror points towards the area to be studied. Two other positions, known as calibration positions, correspond to positions in which the mirror reflects the radiation of reference sources. These reference emitters consist for example of black bodies or a sighting towards cold space. In this case, the calibration of the sensor requires positioning of the mirror in the first calibration position, then in the second calibration position, and finally in the scene position.
It will be appreciated that one of the elements which affects the behaviour of the mechanical positioning device is the actuator. In general, this is an electric motor. Different types of electric motors can be used. Mobile coil motors known as “voice coil” motors have great precision. However, they need control by means of servo-control in a closed loop, and the retention in a given position necessitates a permanent electrical supply. Piezoelectric actuators also have great precision. However, they must also be controlled by means of servo-control in a closed loop. In addition, they have a product of force to course which is relatively low. In practice it is therefore necessary to use piezoelectric actuators with large dimensions. Step motors have the advantage of being able to be controlled without a closed loop, and permit retention of a position without needing a supply. On the other hand, the angular precision is generally insufficient. One solution consists of associating a speed reducer with the step motor. The reducer makes it possible to reduce the angular displacement at the output relative to the angular displacement of the motor. Thus, for each step of the motor, the angular displacement at the output represents a fraction of this step. The reducer can for example take the form of a device with gears. However, in order to obtain low transmission ratios, for example of approximately a hundredth, the device must comprise a large number of toothed wheels. In addition to the problems of complexity and size, a reducer of this type introduces play and resistant torque. Devices for elimination of play exist, but they introduce additional torque. Another reducer solution is based on the use of an arm which is rotated by the motor by means of two connecting rods. The arm is in pivot connection with a frame. A first connecting rod is rotated by the motor. The second connecting rod is connected to the first connecting rod by a first pivot connection, and it is connected to the arm by a second pivot connection. When the axes of these two pivot connections are on the same plane as the axis of rotation of the motor, the two connecting rods generate a dead centre, i.e. a configuration in which the movement of rotation of the arm is inverted. This inversion is accompanied by a localized decrease in the transmission ratio between the angular displacement of the arm and that of the rotor. However, a device of this type comprises only two dead centres, and it is not suitable for angular positioning according to three distinct positions.